When a radio station obtains a broadcasting license from the Federal Communications Commission (FCC) the radio station is allocated a specific frequency range in which it is authorized to transmit to the public. The FCC allocates each FM station a frequency band for transmitting its signals that is 200 kilohertz wide. The radio station typically designates the center of its allotted frequency range as "the" frequency at which it transmits for tuning purposes. A typical FM radio station, for example, might advertise that it broadcasts at 100.1 megahertz. Deviations above and below this center, carrier frequency encode information onto the FM signal. The FCC allotted frequency range for an FM radio station extends 100 kilohertz above and below the 100.1 megahertz carrier frequency.
FM stations that allocate large portions of their on air time to music typically broadcast stereophonically due to the commonly accepted notion that music sounds better when it is heard in stereo. Stereo broadcasting is accomplished by combining the right and the left channel audio content into the transmitted signal and then decoding the transmitted signal at the radio receiver.
A standardized system of FM stereo broadcast signal encoding has developed using a 53 kilohertz frequency band that is used to modulate the carrier signal. The 53 kilohertz band contains three distinct signals which are known as the "composite stereo signal". A left channel plus right channel stereo signal is assigned to a frequency range from 50 hertz to 15 kilohertz. A second range from 23 kilohertz to 53 kilohertz is occupied by the left minus right stereo signal. A pilot tone is generated at a frequency of 19 kilohertz.
The composite stereo signal is combined with the carrier signal (100.1 megahertz for example) by an FM exciter and fed to a signal transmitter which generates a broadcast signal and transmits this broadcast signal to a broadcast antenna. When a remotely located radio tuner receives this stereophonically encoded broadcast signal, tuner circuitry extracts the right and left channel signals from the transmitted composite stereo signal and converts this signal to an audible frequency signal which drives the radio speaker or speakers through an amplifier. The theory of encoding stereo separation information within a 53 kilohertz frequency band is disclosed in U.S. Pat. No. 3,257,511 to Adler et al., issued June 21, 1966.
Radio stations broadcasting music, for example, rock music, often seek to maximize "loudness" of their signals as reproduced by radio receivers tuned to their frequency. This is accomplished by maximizing the station's signal modulation. Signal modulation constraints imposed by the FCC limit the total energy the station can transmit so that the station does not interfere with other stations having the same broadcast frequencies in other areas of the country. Station efforts to attract a larger listening audience by operating above FCC imposed signal strength limits do occasionally occur but this practice is a risk since the FCC can impose fines or even revoke the station's license.
Competition among radio stations for listening audience share has lead to the common practice of clipping the composite signal to enhance loudness. If used sparingly, composite clippers do enhance loudness without adverse effect on the composite stereo signal. Over zealous clipping, however, distorts the main channel (50 hertz-53 kilohertz) modulation signal and also results in cross talk into the 53-100 kilohertz region.
One practice that radio stations have developed to utilize their entire allocated frequency range is to sublease a portion or portions of the unoccupied 47 kilohertz band above the composite stereo signal band for use by paging systems, beepers, and specialized communications such as private security systems. This additional authorized use of the radio station's allocated frequency band is generically referred to as a subchannel carrier authorization or SCA.
Spill over outside the main channel into the 47 kilohertz band (53-100 kilohertz) does not contribute to the information content of the radio broadcast but can adversely impact SCA communications. Additionally, the FCC regulated allowable power transmission covers the entire allocated frequency range so that spurious signals in the 53-100 kilohertz band, caused for example by excessive clipping, constitutes part of the station's allowable transmitted power output. If the radio station operates at maximum permissible power a small but meaningful percentage (5% or more) of the power is wasted due to spurious signal transmission.